For reforming units of steam reforming a starting gas such as a city gas, LPG, a natural gas or the like, a reforming unit described, for example, in WO 00/63114 is known. This reforming unit is one that is used to prepare a reformed gas of a high hydrogen concentration mainly used in a polymer electrolyte fuel cell and as shown in FIG. 5, a burner (70) is disposed at the center of a plurality of cylindrical tubular bodies (61˜69) wherein a combustion gas passage (71), a preheating layer (72), a reforming catalyst layer (73), a heat recovery layer (74), a CO shift catalyst layer (75), a CO removal catalyst layer (78) and the like are formed in spaces of the tubular bodies around the burner (70), respectively. However, such a reforming unit has the following problems (1)˜(3) and has to be further modified.
(1) This reforming unit needs a heat insulation layer (79), a cooling mechanism (80) and the like in the inside thereof and thus, not only the structure becomes complicated, but also the internal thermal performance is low owing to the fact that because the heat insulation layer (79) and the cooling mechanism (80) are, respectively, interposed between these catalyst layers, the respective catalyst layers are separated from one another and are not contiguous, thereby causing the unit to be delayed in temperature rise upon startup and having the startup time prolonged in practice.
(2) Where a Cu—Zn-based CO shift catalyst is used, for example, as a CO shift catalyst, the Cu—Zn-based CO shift catalyst is so low in heat resistance that for continuous use of this catalyst, it is essential to provide the heat insulation layer (79), the cooling mechanism (80) and the like around the CO shift catalyst layer (75) and suppress the temperature of the CO shift catalyst layer (75) to 300° C. or below. More particularly, the reforming catalyst layer (73) has a temperature of 700° C. or over upon reaction, under which if the heat insulation layer (79) or the cooling mechanism (80) is not provided between the reforming catalyst layer (73) and the CO shift catalyst layer (75), then the temperature of the CO shift catalyst layer (75) is elevated via heat transmission from the reforming catalyst layer (73), resulting in the temperature of the filled CO shift catalyst exceeding its heat-resistant temperature.
(3) Because the usable temperature of the CO shift catalyst layer (75) is limited to 200˜300° C., the reaction velocity caused by the catalyst is so low that a large amount of the CO shift catalyst is required, which renders the unit large in size, thereby increasing the weight correspondingly.
In cases where a limitation is not placed on such a reforming unit as set out hereinabove but a reforming unit is employed for fixed type purposes (residential PEFC applications) or for automobiles, it is essential that a reforming system including a reforming unit be small in size and light in weight as a whole. Additionally, various improvements are necessary to make a high efficiency in the practical service conditions, not to mention a startup time upon commencement of operation, or to realize a shortening in the startup time.
The invention has been accomplished in view of such problems as set forth above with respect to the steam reforming unit and has for its object the provision of a cylindrical steam reforming unit which is small in size and light in weight, has good startup characteristics, can be operated at a high thermal efficiency and is able to stably produce hydrogen.